A TOFMS is an apparatus for analyzing the composition of a target by irradiating the target with a primary beam so as to eject particles from the target which are ionized and then directed as a secondary beam through a "drift" region to a detector. The target in some applications is the surface of a solid or liquid and in other applications is a gas injected into an ionization chamber. The velocity of various particles is proportional to the square root of the mass of the particle so that the composition of the drifting beam can be determined by measuring the time required by each species of particle to reach the detector.
Interpretation of the data is complicated by the fact that heavy particles released from the target at one instant must be distinguished from light (faster) particles released at a later instant. One approach to distinguishing between light and heavy particles is accomplished by applying the primary beam in pulses with sufficient time between pulses to enable all of the particles from one pulse to reach the detector before the following pulse of the primary beam. This places a severe limitation on the rate of data generation and sample throughput. The duration of the pulse must be severely limited in order for this approach to be effective.
Another complication arises from the dispersion of kinetic energies of particles from the same species released by the same pulse due to localized variations in conditions of sputtering and desorption from surface of the target and the angular dispersion of particles ejected from the target surface. This condition degrades the resolving power of the apparatus. A number of disclosures have appeared which are intended to make simultaneous the arrival at the detector of all particles from the same species from the same pulse and to compensate for energy and angular differences.
For example, U.S. Pat. No. 5,376,788 to Standing discloses a TOF mass spectrometer with resolution enhanced by producing electrical modulation of the kinetic energy imparted to the generated ions.
U.S. Pat. No. 5,128,543 to Reed discloses a TOFMS analyzer featuring two or more particle steering analyzers for compensating for the energies of same species particles thereby improving resolution. The three spherical steering analyzers rely on differentiating centripetal forces between the particles of same species but slightly different energies to redirect the path of the secondary beam by 270 degrees onto a detector plate.
TOFMS has been adapted to investigate targets which are gaseous and targets which are the surfaces of solid or liquid samples.
In the case of surfaces of solid samples, the technique has been extended to rastering the the primary beam over the target surface to accomplish individual localized analysis which can be displayed as an image or map of the lateral composition of the sample.
For example, U.S. Pat. No. 4,983,831 to Migeon discloses positioning deflector plates in the drift region to which a deflecting voltage to the secondary beam is applied. The secondary particles are discriminated by deflecting them at an angle which is variable periodically such that particles having a given time of flight are deflected in a predetermined direction irrespectively of the point on the target from which they have been liberated. Then the secondary particles moving in the predetermined direction are selectively detected. A limitation of this device is that only one species is detected.
The detector sensing the signal from the secondary beam (which is focused on a single detection location) is coupled to a CRT which translate the detected signal vs. time into a map on the CRT screen of the distribution of a single species on the target surface.
Other detection constructions are known in which a secondary particle originated from a location of an irradiated or illuminated target is mapped directly onto a surface of a detector. One such system uses a "position sensitive detector" which is available in several forms.
In one such form, the detector comprises a bundle of parallel capillary tubes with ends of the tubes forming the front detector surface. A beam of arriving secondary ions strike the inside surface of tubes in a localized area which are specially treated to generate electrons by secondary emission. The intensity of the secondary electrons is amplified as they travel to the far end of the tubes. At the rear surface of the array of tubes, the arrival is detected by a means which encodes the position of ion beam arrival. A direct indication of the intensity of the ion beam vs. illuminated sample position is available thereby.
Other disclosures have been published describing the use of deflection plates to improve resolution.
U.S. Pat. No. 5,347,126 to Krauss discloses injection of an ion beam into a pair of deflection regions separated by a drift space. The deflection regions include aperture plates such that pulses applied to deflection plates in the deflection regions cut off the forward and rearward end of the ion beam.
U.S. Pat. No. 5,300,774 to Buttrill discloses a a time of flight mass spectrometer in which a barrier defines an aperture in the path of the ion beam positioned to block ions having an extra large deviation of time of flight.
Disclosures have been published regarding approaches to increase rate of data throughput that is inherently limited in state of the art TOFMS apparatus by the time of flight difference between light and heavy particles.
U.S. Pat. No. 5,331,158 to Dowell discloses generating two secondary beams in tandem, each beam directed toward its own detector In one embodiment, two sources of primary beams are used, each generating its own secondary beam. In another embodiment, the primary beam is alternately directed in two separate directions by deflection in the ionizing chamber. Data generated by one primary beam is generated while the other primary beam is shut off The system is adapted to investigating gas sample targets injected into the ionization chamber. The construction requiring one primary beam for each secondary beams such as with a plurality of primary beam sources or even the the approach of deflecting the primary beam severely limits the number of discrete secondary beams that can be generated.
Various methods have been disclosed for preparing target surfaces for examination by TOFMS and each of these methods can present unique problems to implementing the TOFMS technique. For example, U.S. Pat. No. 5,360,976 to Young discloses preparation of a target surface by admitting a species to be examined as a gas into an evacuated ionization chamber having a cooled target surface so that the gas molecules are absorbed on the target surface. The molecules are then desorbed by bombardment with a primary beam. This technique is limited by the length of time that would be available before the supply of molecules is depleted.